Question

I have some Guice binding code using generics that compiles and functions fine from Eclipse’s compiler, but not from the Java (command-line) compiler. I upgraded to the latest (1.7.0_01) Java SDK but still get the following error.

[error] ...\BindCategorySelectorActivity.java:42: error: inconvertible types
[error]                                 (Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>) CategoryDataProvider.class);
[error]                                                                                                                             ^
[error]   required: Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>
[error]   found:    Class<CategoryDataProvider>
[error] 1 error
[error] {file:/.../compile:compile: javac returned nonzero exit code

Relevant code:

public interface Category extends DatabaseItem {}
public class CategoryDataProvider implements 
 ListAdapterDataProvider<Row<Category>> {}
public class BindListViewHandlerWithSpecificProvider extends AbstractModule {
    public BindListViewHandlerWithSpecificProvider(
     Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>
      dataProviderClass) {}
}

@SuppressWarnings("unchecked")
// Error happens here:
final BindListViewHandlerWithSpecificProvider 
 bindListViewHandlerWithSpecificProvider = 
  new BindListViewHandlerWithSpecificProvider(
   (Class<? extends ListAdapterDataProvider<Row<? extends DatabaseItem>>>)
    CategoryDataProvider.class);

Answer 1

Do yourself a favor and do an upcast followed by a downcast:

Class<...> foo = (Class<...>)(Object)MyClass.class;

The issue is that CDP.class is of type Class<CDP>, CDP being a raw type. While a parameterized type C<T1,...,Tn> is the subtype of the raw type C (§4.10.2), the inverse is not true: C is not a subtype of C<T1,...,Tn>. This only appears to be true due to unchecked conversion (§5.1.9). This is causing your issue: You expect CDP to “extend” (as in the upper bound of Class<? extends ...>) LADP<Row<? extends DI>>. This is not the case because type argument containment (§4.5.1.1) is defined over subtyping and does not consider unchecked conversion.

(Or to cut to the chase: javac has got this one right.)